A. Wireless Location
Early work relating to Wireless Location Systems is described in U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using time difference of arrival (TDOA) techniques. This and other exemplary patents (discussed below) are assigned to TruePosition, Inc., the assignee of the present invention.
The '144 patent describes what may be referred to as an uplink-time-difference-of-arrival (U-TDOA) cellular telephone location system. The described system may be configured to monitor control channel transmissions from one or more cellular telephones and to use central or station-based processing to compute the geographic location(s) of the phone(s). For example, in station-based processing, which may be employed for reverse control channel signal detection, cross-correlations are performed at the cell sites (or signal collection systems) in the following manner: For each “strong” signal, which may be considered a reference signal, received on a particular control channel at a particular first cell site, that strong signal is first applied to a signal decoder, such as that used by the cellular system itself. This decoder demodulates the cellular signal to produce the original digital bit stream which had been modulated to produce the cellular signal. This digital bit stream is then modulated by the cell site system to reconstruct the original signal waveform as it was first transmitted by the cellular telephone. This reconstructed signal waveform is cross-correlated against the received signal at the first cell site. The cross-correlation produces a peak from which an exact time of arrival can be calculated from a predetermined point on the peak. The first cell site system then sends the demodulated digital bit stream and the time of arrival to the central site over the communications line. The central site then distributes the demodulated digital bit stream and the exact time of arrival to other cell sites likely to have also received the cellular transmission. At each of these other second, third, fourth, etc., cell sites, the digital bit stream is modulated by the cell site system to reconstruct the original signal waveform as it was first transmitted by the cellular telephone. This reconstructed signal waveform is cross-correlated against the signal received at each cell site during the same time interval. The cross-correlation may or may not produce a peak; if a peak is produced, an exact time of arrival (TOA) can be calculated from a predetermined point on the peak. This TOA is then sent to the central site, and a delay difference, or TDOA, for a particular pair of cell sites can be calculated. This method permits the cell site systems to extract TOA information from an extremely weak signal reception, where the weak signal may be above or below the noise level. This method is applied iteratively to sufficient pairs of cell sites for each strong signal received at each cell site for each sample period. The results of the delay pairs for each signal are then directed to the location calculation algorithm.
TruePosition and others (e.g., KSI, Inc.) have continued to develop significant enhancements to the original inventive concepts. Some examples are discussed below.
U.S. Pat. No. 6,047,192, Apr. 4, 2000, “Robust, Efficient, Localization System,” is another example of a prior art patent describing a similar process (referred to as “matched-replica processing”) for processing mobile transmitter signals to determine location related signal parameters, which may be employed to calculate the transmitter location.
Another exemplary prior art patent is U.S. Pat. No. 6,091,362, Jul. 18, 2000, “Bandwidth Synthesis for Wireless Location System.” This patent describes a system and process offering improved accuracy of location information and greater time resolution. In the described system, signals transmitted by wireless telephones are received at a plurality of signal collection sites. To improve the accuracy of the location information, the system synthesizes greater bandwidth, and thus greater time resolution, than would otherwise be available. The location system can issue commands to cause a wireless transmitter to be located to change frequency channels, and a doubly-differenced carrier phase of the transmitted signal, or the TDOA, is observed at each of many frequencies spanning a wide bandwidth. The phase-measurement data from these many frequencies are combined to resolve the inherent integer-wavelength ambiguity. The invention may be utilized to obtain a bandwidth greater than the typical bandwidth of the signals to be cross-correlated (in either the time or frequency domains) in a cellular telephone location application.
Another example is U.S. Pat. No. 6,646,604, Nov. 11, 2003, “Automatic Synchronous Tuning of Narrowband Receivers of a Wireless Location System for Voice/Traffic Channel Tracking.” This patent describes a transmitter locating method that involves performing location processing on signals received during an automatic sequential tuning mode of operation, wherein narrowband receivers are tuned sequentially and in unison to a plurality of predefined RF channels. Signal transmissions of interest in these channels are digitally recorded and used in location processing. The identity of the located transmitter(s) is determined by matching a location record to data indicating which wireless transmitters were in use at a time corresponding to the location record, and which cell sites and RF channels were used by each wireless transmitter.
An example of a wireless location system (WLS) of the kind described above is depicted in FIG. 1. As shown, the system includes four major subsystems: the Signal Collection Systems (SCS's) 10, the TDOA Location Processors (TLP's) 12, the Application Processors (AP's) 14, and the Network Operations Console (NOC) 16. Each SCS is responsible for receiving the RF signals transmitted by the wireless transmitters on both control channels and voice channels. In general, an SCS (now sometimes called an LMU, or Location Measuring Unit) is preferably installed at a wireless carrier's cell site, and therefore operates in parallel to a base station. Each TLP 12 is responsible for managing a network of SCS's 10 and for providing a centralized pool of digital signal processing (DSP) resources that can be used in the location calculations. The SCS's 10 and the TLP's 12 operate together to determine the location of the wireless transmitters. Both the SCS's 10 and TLP's 12 contain a significant amount of DSP resources, and the software in these systems can operate dynamically to determine where to perform a particular processing function based upon tradeoffs in processing time, communications time, queuing time, and cost. Each TLP 12 exists centrally primarily to reduce the overall cost of implementing the WLS. In addition, the WLS may include a plurality of SCS regions each of which comprises multiple SCS's 10. For example, “SCS Region 1” includes SCS's 10A and 10B that are located at respective cell sites and share antennas with the base stations at those cell sites. Drop and insert units 11A and 11B are used to interface fractional T1/E1 lines to full T1/E1 lines, which in turn are coupled to a digital access and control system (DACS) 13A. The DACS 13A and another DACS 13B are used in the manner described more fully below for communications between the SCS's 10A, 10B, etc., and multiple TLP's 12A, 12B, etc. As shown, the TLP's are typically collocated and interconnected via an Ethernet network (backbone) and a second, redundant Ethernet network. Also coupled to the Ethernet networks are multiple AP's 14A and 14B, multiple NOC's 16A and 16B, and a terminal server 15. Routers 19A and 19B are used to couple one WLS to one or more other Wireless Location System(s).
B. Evolving Wireless Standards and Air Interface Protocols
Over the past few years, the cellular industry has increased the number of air interface protocols available for use by wireless telephones, increased the number of frequency bands in which wireless or mobile telephones may operate, and expanded the number of terms that refer or relate to mobile telephones to include “personal communications services,” “wireless,” and others. The air interface protocols now used in the wireless industry include AMPS, N-AMPS, TDMA, CDMA, GSM, TACS, ESMR, GPRS, EDGE, UMTS WCDMA, and others. UMTS is a wideband CDMA air interface protocol defined by ETSI 3GPP. This protocol is similar to the CDMA protocols in EIA/TIA IS-95, or CDMA 2000, but does not require synchronization of the base stations, and also provides a high level of interoperability with GSM network infrastructure.
Orthogonal Frequency Division Multiplexing (OFDM) is a multiplexing technique in which a given subscriber may be assigned many frequency channels over which it will simultaneously transmit. The multiplexing scheme provides high bandwidth efficiency and broadband wireless communication in a high multi-path environment. WiFi as defined in IEEE 802.11 and WiMax as defined in IEEE 802.16 utilize OFDM. It is expected that IEEE 802.20 (when re-ratified) will utilize OFDM.
Uplink TDOA location of fourth generation (4G) broadband OFDM signals with bandwidths that can exceed 20 MHz requires expensive receiver and signal processing hardware. The SCSs (or LMUs) may be required to receive, sample, store and process these broadband signals, making the hardware significantly more expensive than what is required for third generation (3G) signals, such as UMTS or CDMA 2000 WCDMA signals occupying a bandwidth of 3-5 Mhz. As described in greater detail below, a goal of the present invention is to provide a way to accomplish U-TDOA location on the broadband 4G waveforms by collecting and processing only a portion of the transmitted signal, reducing the required bandwidth, memory, and digital signal processing required in the SCS/LMU, while still achieving high accuracy.